The present invention relates to a radio receiver which may be used in a mobile communication base station system, for example, and in particular, to a high sensitivity radio receiver which exhibits an improved reception sensitivity achieved through the cooling of a radio frequency receiver section.
An arrangement of a conventional high sensitivity radio receiver is shown in FIG. 1 as comprising an antenna 1 including n (where n=4 in FIG. 1) antenna elements 1a, 1b, 1c, 1d; a plurality of element feeders 2a, 2b, 2c, 2d (collectively referred to as xe2x80x9celement feeder 2xe2x80x9d) each connected to the antenna elements 1a, 1b, 1c, 1d, respectively; a phase shifter 3 which adjusts phases of input signals from the individual element feeders 2a, 2b, 2c, 2d and combines them; an antenna feeder 4 for transmitting an output signal from the phase shifter 3; a receiving filter 5 for selecting and passing signals in a desired reception band from the signal transmitted through the antenna feeder 4; a low noise reception amplifier 6 for providing a low noise amplification of an output signal from the receiving filter 5 to a desired level; and an output terminal 7 for delivering an output signal from the low noise reception amplifier 6. Both the receiving filter 5 and the low noise reception amplifier 6 are confined in a cryostat 8, and are cooled down by cooling means 9 having a cold stage 9a which is effective to cool down the receiving filter 5 and the amplifier 6. By way of example, the cooling means 9 may include a cold head to which a copper plate may be attached to serve as the cold stage 9a, on which the receiving filter 5 and the low noise reception amplifier 6 can be mounted. There are also provided a first power supply terminal 10 which feeds the low noise reception amplifier 6 and a second power supply terminal 11 which feeds the cooling means 9. The cryostat 8 and the cooling means 9 are contained in a casing 12. Reference may be made to xe2x80x9cA Receiver Front End For Wireless Base Stationsxe2x80x9d in Microwave Journal, Vol .39, No. 4, April 1996, for a high sensitivity radio receiver containing a receiving filter and a low noise reception amplifier, both of which are cooled, and to MWE ""97 Microwave Workshop Digest, PS4-5, xe2x80x9cLong Life Small Cryo-Coolerxe2x80x9d, for example, for a cooling means which may be used to cool down the receiving filter and the low noise reception amplifier.
The phase shifter 3 may be arranged as shown in FIG. 2. The antenna elements 1a, 1b, 1c, 1d are disposed in a vertical array with a spacing d between the elements, and a radio wave having a wavelength of xcex which is incident on the array with an angle of depression of xcex8 with respect to the perpendicular to the array may reach the antenna array with a phase difference between the adjacent antenna elements as indicated below.
2xcfx80d sin xcex8/xcexxe2x80x83xe2x80x83(1)
It will be noted that the upper the location of a particular element in the array, the more the phase of the received wave is delaying. Accordingly, when the phase shifter 3 synthesizes the signals received by the antenna elements in a manner such that a successively increasing phase delay, in increment of a xcex94xcfx86=2xcfx80d sin xcex8/xcex, is applied to signals received by the antenna elements 1b, 1c, 1d as referenced to the signal received by the antenna element 1a, the signals received by the individual antenna elements are combined at an equal phase relative to each other to provide a maximum reception strength for the incidence of the radio wave at the angle of depression of xcex8, thus directing a main lobe of the antenna directivity, hereafter the referred to as antenna beam, in the direction of the angle xcex8. In this manner, a design may be employed in the mobile communication base station system which directs the center of the antenna beam depressed toward the ground surface or downward in order to enhance the sensitivity for radio waves transmitted by mobile units which are resident in a service area of a mobile communication base station. While not shown in FIG. 1, where the antenna 1 is also used for signal transmission, the transmission antenna beam may be directed toward the ground surface in order to reduce interferences with radio waves of adjacent areas.
The receiving filter 5 and the low noise reception amplifier 6 are confined in the cryostat 8 which is arranged to provide an isolation from external heat input as by vacuum heat insulation, for example. The cooling means 9 comprises a cryogenic refrigerator, which may be commercially available, and which is capable of maintaining the receiving filter 5 and the low noise reception amplifier 6 at a temperature which may be as low as several tens of Kelvin degrees, for example, for a prolonged length of time in a stable manner.
When the receiving filter 5 and the low noise reception amplifier 6 are cooled down to a cryogenic temperature for a prolonged length of time in a stable manner, thermal noises generated in the receiving filter 5 and the low noise reception amplifier 6 are reduced to a minimum while allowing the insertion loss of the receiving filter 5 to be reduced. As a consequence, the noise figure of the receiver shown in FIG. 1 is greatly improved as is the reception sensitivity. Accordingly, the use of the high sensitivity radio receiver shown in FIG. 1 brings forth advantages that a reception output in excess of a minimum prescribed C/N (carrier power/noise power), for example, can be obtained from a low level received signal and that a less power is required on the transmitting side to achieve a reception output in excess of a minimum prescribed C/N.
A conventional high sensitivity radio receiver utilizes the phase shifter 3 for adjusting the direction of the antenna beam, and thermal noises generated in the phase shifter 3 degrade the reception sensitivity of the receiver. The phase shifter 3 is disposed outside the casing 12, and this requires the provision of the antenna feeder 4, the loss of which in turn degrades the reception sensitivity disadvantageously. It is an object of the invention to provide a high sensitivity radio receiver which exhibits a high reception sensitivity if the phase shifter 3 is used for the purpose of adjusting the direction of the antenna beam.
In accordance with the invention, an antenna comprises an array of n antenna elements where n is an integer equal to or greater than 2. Signal received by individual antenna elements are input to a phase shifter where their phases are adjusted to define the center of an antenna beam which is directed in a desired direction before they are synthesized to provide an output signal, which is then passed through a receiving filter. A filter output is amplified in a low noise reception amplifier to be delivered to an output terminal. It is premised that the receiving filter and the low noise reception amplifier of the high sensitivity radio receiver are confined in a cryostat, and are cooled down by cooling means.
It is a first feature of the invention that the phase shifter is confined in the cryostat to be cooled down by the cooling means.
An antenna duplexer is inserted in a path between the phase shifter and the receiving filter, both of which are confined in the cryostat, and a transmitted signal is passed through the antenna duplexer to be delivered to the antenna.
The antenna duplexer comprises a receiving filter which passes a reception band while eliminating a transmission band and a transmitting filter which passes the transmission band while eliminating the reception band. At least the receiving filter of the antenna duplexer is confined in the cryostat to be cooled down by the cooling means.
Antenna duplexers are inserted each in a path between one of the n antenna elements and the phase shifter. A transmitted signal is divided into n signals in a transmitting phase shifter where the relative phases of the divided signals are adjusted to provide output signals, which are then passed through the n antenna duplexer to be delivered to the antenna elements.
It is a second feature of the invention that the receiving filter is formed of a superconductor material which assumes a superconducting condition when it is cooled down by the cooling means. At least part of the receiving filter of the antenna duplexer is confined in the cryostat and is connected before the receiving filter formed of the superconductor material which is, cooled down by the cooling means for as to be also cooled by the cooling means. The receiving filter of the antenna duplexer is formed of a non-superconductor material.
It is a third feature of the invention that a second receiving filter is connected after the low noise reception amplifier and is confined in the cryostat to be cooled down by the cooling means.
It is a fourth feature of the invention that each of the n antenna elements is connected to a respective antenna duplexer, the output of a respective receiving filter of the duplexer of which is connected to a series circuit including a phase shifter, a receiving filter, and a low noise reception amplifier. The outputs of these n series circuits are connected together and connected to the output terminal. The n series circuits are confined in the cryostat to be cooled by the cooling means.